1. Field of the Invention
The present invention relates to a failure detecting apparatus for a hydraulic circuit of a gearbox.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 11-236965 discloses a belt-type continuously variable transmission that has an endless belt wound around a driving pulley provided on an input shaft and a driven pulley provided on an output shaft and controls with a linear solenoid valve the pulley hydraulic pressure of the hydraulic oil supplied to a hydraulic oil chamber of the driving pulley and a hydraulic oil chamber of the driven pulley in order to control the transmission gear ratio in a continuously variable manner by increasing or decreasing the effective diameter of pulleys.
The hydraulic circuit of the belt-type continuously variable gearbox includes a regulator valve that reduces the discharge pressure of an oil pump to line pressure, a clutch reducing (CR) valve that reduces the line pressure to CR (clutch reducing) pressure, which is the operating pressure of a hydraulic clutch and the pressure source of the linear solenoid valve, and a modulator valve that reduces the CR pressure to modulator pressure, which is the pressure source of an operating signal pressure of each shift valve.
When a closed failure occurs at the modulator valve, the closed failure can be detected since the operation of the subject to be controlled becomes abnormal because the operating signal pressure of which the pressure source is the modulator pressure disappears due to the modulator pressure being reduced to zero. On the other hand, when an open failure occurs at the modulator valve, the open failure cannot be detected since the subject to be controlled operates normally even though a high modulator pressure, which is higher than a normal value, is output.
When an open failure occurs in the modulator valve, as described by embodiments, when a control system failure occurs, lateral pressure applied to the driving pulley and the driven pulley is excessively reduced, causing the endless belt to slip and, as a result, lowering the durability of the belt-type continuously variable gearbox. For this reason, it is necessary to reliably detect an open failure of the modulator valve.